1. Field of the Invention
This invention relates to a power supply circuit and an electronic control unit including the same.
2. Description of the Prior Art
Electronic control units (ECUs) for a motor vehicle may have a power supply circuit for supplying a backup power from a backup battery to the RAM therein to hold the data while the main power is OFF. In such an ECU, because the capacity of the backup battery is not so large, reduction in power consumption in the power supply circuit is required.
FIG. 4 is a schematic circuit diagram of a prior art power supply circuit 1 for generating and supplying a backup power from a backup battery (not shown) to the RAM (not shown) in the electronic control unit (not shown) to hold the data while the main power is OFF. This power supply circuit 1 has a series regulator structure. That is, the base of the output power transistor Q1 is controlled by an output of an operational amplifer 7 which represents the difference between a reference voltage Vr and the detection output voltage Vd which is derived by voltage-dividing the output voltage Vo of the output power transistor Q1 to make the output voltage constant. The power supply circuit 1 further includes an overheat detection circuit 8 for detecting overheat which may be caused by a relatively large magnitude of the output current of the power supply circuit 1. The overheat detection circuit 8 is supplied with a bias current from a constant current source 9. Here, the bias current is always supplied to the overheat detection circuit 8, so that the power consumption in the overheat detection circuit 8 and the accompanying circuits is negligible. Therefore, suppress in the power consumption in the power supply circuit for backup for the RAM is further required.
Another aim of the present invention provides a power supply circuit capable of suppression of current for error detection and a superior electronic control unit including the same.
In accordance with one aspect of the present invention, a magnitude of a current flowing through a power transistor in the power supply circuit is detected, and an error condition which may be caused by a relatively great magnitude of the current is detected by an error detection circuit, which is operated when the magnitude of the current is greater than a threshold value to suppress power consumption.
Another aspect of the present invention provides a power supply circuit capable of suppressing power consumption, wherein the transistor supplies a current to a load, a current detection circuit detects a magnitude of the current, an error detection circuit detects an error condition which may be caused by a relatively great magnitude of the current and outputs the detection result; and a control circuit operates the error detection circuit when the magnitude of the current is greater than a threshold value.
The control circuit may include a bias circuit for supplying a bias current to the error detection circuit to operate the error detection circuit when the magnitude of the current is greater than the threshold value.
The current detection circuit may include a resistor connected in series with the transistor and the load; and a transistor turning ON on the basis of a voltage drop between the resistor.
The error detection circuit may detect overheat in the power supply circuit as the error condition.
The control circuit may include a bias circuit for supplying a bias current to the error detection circuit to operate the error detection circuit when the magnitude of the current is greater than the threshold value.
The transistor may include a multi-collector structure including a first collector supplying the current and a second collector supplying a detection current corresponding to the current to detect the magnitude of the current, and wherein the current detection circuit includes the second collector.